This invention relates to an improvement in internal combustion engines, particularly engines for motor vehicles.
In conventional internal combustion engines for motor vehicles, since the opening of the throttle valve is low and the amount of sucked air is small during idling and operation of the engines under light loads, the velocity of flow of the air-fuel mixture entering the cylinders from the intake manifold is low during the suction stroke of the engine, thus the swirl produced in the cylinder is weak. As a result, during ignition, which normally takes place at the end of the compression stroke of the engine, the swirl of the mixture retained in the cylinder becomes weaker, so that firing and combustion of the mixture are not adequate. To ensure stable operation of the engine, it is necessary to supply a mixture having an air-fuel ratio which is lower than that of the mixture supplied during medium or high loaded driving of the engine; that is, a richer mixture is required. This results in an increase in fuel consumption as well as an increase in the amounts of carbon monoxide (CO) and hydrocarbon (HC) contained in exhaust gas due to imperfect combustion of the richer mixture.
Recently it has been proposed to burn a mixture much leaner than the stoichiometric mixture in order to reduce the amount of CO and HC, and particularly, the amount of nitrogen oxides (NO.sub.x) contained in the exhaust gas. Also, it has been proposed to draw a portion of exhaust gas from the exhaust system of the engine and to mix and burn it with the mixture in the cylinder to reduce the amount of NO.sub.x in the exhaust gas. However, in both cases, firing and combustion of the mixture become inefficient so that drivability and rate of fuel consumption become worse.